(1) Field of the Invention
This invention relates to a process for the preparation of an electron beam curing gypsum panel, or more particularly relates to a process for the preparation of an electron beam curing gypsum panel, particularly electron beam curing gypsum tiles, which comprises subjecting a hydrated gypsum slurry containing or free of an additive, fibrous material, fine aggregates etc, to defoaming, setting and drying to form a gypsum substrate, coating an electron beam curing resin composition on the surface of the gypsum substrate to form a coated film, and irradiating accelerated electron beam on the coated film to be cured.
(2) Description of the Prior Art
Gypsum panels have long been known, particularly gypsum tiles have been used for interior decoration in Portugal, and is similar in design to fresco painting which is the art of painting on a lime plaster surface with colors ground in water or a limewater mixture and is applied to the plaster wall.
Many attempts have been made on the process for the preparation of the gypsum tile by use of gypsum, and there have been provided various molded gypsum products such as those obtained by casting a hydrated gypsum slurry into a ceiling or wall and by inserting a stainless steel mesh into a core material on setting for molding to be reinforced; those obtained by transferring a gravure printing to a gypsum slurry reinforced with glass fiber by means of transferring process, drying the slurry, coating for penetration thereon an ultraviolet radiation curing coating composition, and by irradiating thereon ultraviolet light to be cured; a glass fiber-reinforced gypsum board (GRG) made from glass fiber and gypsum slurry; a foamed gypsum board formed by incorporating a cellulose material and glass fiber into a foamed gypsum composition; and the like. As described above, the application and consumption of molded gypsum products are gradually being increased in the field of architecture.
As an example of the coating method of a porous substrate such as an aerated concrete panel for the purpose of coating on the aerated concrete panel, to which waterproof coating is difficult to be applied, to form a completely waterproof coated film thereon, Japanese laid-open patent application No. 17431/1974 discloses a method of coating an aerated concrete panel which comprises coating a curable coating composition onto an inner surface of a mold, to which one face of the concrete panel is slightly recessed, allowing a resulting coating to be half cured, and pressing the surface of the concrete panel to the coating to be cured, the coating composition being transferred onto the concrete panel. Although the Japanese laid-open patent application as above discloses that the curable coating composition is preferably those curable by heating, and may be those curable by the irradiation of ultraviolet light or accelerated electron beam, there is described that the curable coating composition of the latter type of curing is practically difficult to be applied due to the presence of the mold, and further no examples in the aforesaid Japanese patent application refer to the transfer printing process as above by use of ultraviolet radiation or electron beam radiation. The gist of the invention disclosed in Japanese laid-open patent application No. 17431/1974 resides in that the half cured coated film formed on the inner surface of the mold is transferred onto the concrete panel instead of coating a coating composition directly on an aerated concrete panel, and the invention disclosed therein gives no suggestions of preparing gypsum tiles by applying a gypsum substrate formed from a hydrated gypsum slurry having a specified water-gypsum ratio to an electron beam curing coating method according to the present invention.
Japanese laid-open patent application No. 63725/1974 discloses a process for sealing a surface of a porous material such as plastics, FRP, foamed plastics, asbestos fiber material, or fiberboard which comprises applying to the surface a thin film layer of a surface treating agent composed of an ultraviolet radiation curable resin composition, both irradiating ultraviolet light thereon and heating the thin film layer with an intention to seal rapidly and effectively the surface of the porous material. The invention disclosed in the aforesaid Japanese patent application is directed to a process for sealing the surface of the porous material as above, and has a quite different idea from that of the present invention which is directed to a process for preparing a colored tile from a gypsum substrate formed from a hydrated gypsum slurry having a specified water-gypsum ratio. Moreover, the process disclosed in the aforesaid Japanese patent application is quite different from that of the present invention in that a combined application of ultraviolet light and heat is essential to the former process.
Japanese laid-open patent application No. 128927/1974 discloses a method of improving the water resistance of cured gypsum hydrate material which comprises coating a specified ultraviolet radiation curable resin composition on the surface of the gypsum material to be cured. It is an object of the invention disclosed in the aforesaid Japanese patent application to modify a gypsum board, which is liable to absorb water so readily as to be easily degraded mechanical strength, by coating thereon the ultraviolet radiation curable resin, while an object of the present invention is to provide a process for the preparation of an electron beam curing gypsum panel for use in architecture, particularly the pre-finished gypsum tile. Application of an ultraviolet radiation curable resin to the gypsum substrate has such a drawback that the viscosity of the resin becomes higher on coating application compared with the case where an electron beam curing resin is applied. The ultraviolet radiation curing process has many disadvantages compared with the electron beam curing process in that the former process is unsuitable for use in outdoor exposure due to low surface hardness of the cured film and to bad weathering resistance, and that the curable film thickness is small and curing performance is inhibited when an opaque colorant such as pigment is incorporated due to lower penetration of the irradiation energy in the former process. The present inventors made extensive studies in order to overcome the disadvantages of the ultraviolet radiation curing process to find out that the ultraviolet radiation curing process is unsuitable for the preparation of the pre-finished gypsum tile, resulting in developing a novel process for the preparation of an electron beam curing gypsum tile by use of an electron beam curing resin composition.
As electron beam curing gypsum panels have been primarily used as backing substrate, there are some problems to be solved for preparing an electron beam curing gypsum panel. An example of the serious problems to be solved as above is that the adhesive tape of cellophane or cloth base is hardly adhered, or is not adhered at all to the surface of the cured gypsum material when the material contains moisture; even if it is apparently adhered to the surface, there may be only gypsum powder left on adhesive side of the tape after peeling it off, because, microscopically speaking, the surface of the cured gypsum material has such a structure that crystals of gypsum are continuously arranged thereon in a powdered or efflorescence state.
Therefore, when an ultraviolet radiation curable resin composition is coated to be permeated, the cured gypsum material is preheated or the resin is warmed so that permeability of the resin may be increased. In such case, good adhesive properties may be expected if the relationship between the viscosity and depth of permeation of the resin or of the coating composition is controlled. However, there is such a disadvantage that the resin or the coating composition partly remains uncured, because the ultraviolet radiation energy required for curing does not sufficiently reach the deepest part of permeated resin. In order to overcome the aforesaid disadvantage, a combined use of an ultraviolet radiation curable resin and a heat curable resin or an application of multilayer coating, for example, primer coating followed by top coating is needed. There is also such a problem to be solved that a coating composition must be applied to form a transparent film after applying a colored primer coating, because curing step is extremely inhibited when an opaque color pigment is incorporated in the ultraviolet radiation curable resin composition.
The invention disclosed in Japanese laid-open patent application No. 113816/1974 relates to a surface finishing process which comprises coating for penetration a specified monomer-polymer mixture curable by the ionized radiation such as accelerated electron beam on a calcium silicate board, and irradiating thereon said electron beam to be cured. The aforesaid process is in common with the process of the present invention only in the application itself of the electron beam curing step. However, it is an object of the invention disclosed in Japanese laid-open patent application No. 113816/1974 to solve such problems peculiar to the calcium silicate board that the application of a sealer to the calcium silicate board for controlling the extremely high moisture-absorption characteristics and coating permeability of the calcium silicate board results in reduced adhesive properties of the radiation cured coating, as well as difficulties in pre-finishing, while it is an object of the present invention to provide a surface-finished gypsum tile having the physical properties similar to those of the conventional baked tile to be quite different from the aforesaid object of the invention disclosed in Japanese laid-open patent application No. 113816/1974. Moreover, Japanese laid-open patent application No. 113816/1974 suggests nothing about the aforesaid object of the present invention. The present invention is technically different from that of the aforesaid Japanese patent application in that the subject matter of the present invention is directed to a gypsum substrate having a specified water-gypsum ratio, or to a molded substrate formed by adding thereto additives and fibrous materials differently from the calcium silicate board, to which the subject matter of the process of the invention disclosed in the aforesaid Japanese patent application is directed. Moreover, Japanese laid-open patent application No. 113816/1974 suggests nothing about the aforesaid subject matter of the present invention, to say nothing of details of the process of the present invention. Therefore, it is definitely impossible to obtain practically useful electron beam curing gypsum panels, particularly electron beam curing gypsum tiles simply by substituting the steps of the process for the preparation of electron beam curing gypsum panels, particularly gypsum tiles with the surface-finishing method disclosed in the aforesaid Japanese patent application.
Japanese laid-open patent application No. 43410/1973 discloses a process for improving the mechanical strength of a concrete block, in which a liquid potential medium is saturated into the block, then saturated block is cured by heat or by irradiation of ionizing radiation. The process of the invention disclosed in the aforesaid Japanese patent application is fundamentally different from the process of the present invention in that the definition of the medium in the aforesaid Japanese patent application is vague and the chemical composition of the medium is not clear, and in that the medium is saturated into the concrete block at a degree of saturation of from 3 to 25% by weight. Thus, the invention disclosed in Japanese laid-open patent application No. 43410/1973 is definitely different from the present invention in its object, construction as means for achieving the object, and in its effect.
Japanese patent publication No. 35058/1974 discloses a sealing method which comprises impregnating pores of porous rigid objects such as castings and sintered metal parts with a polymerizable anaerobic sealant, for example, a sealant mainly composed of acrylate ester followed by applying thereto a curing treatment. However, the aforesaid Japanese patent apparently suggests nothing about the process for the preparation of gypsum tiles according to the present invention.
The gypsum tile, to which the present invention relates, has such advantages that (1) although it is expanded on setting, its dimensional accuracy is greatly improved in the dry condition when the exothermic reaction has been completed and free water therefrom has evaporated; (2) when white, it has a great adaptability in architectural design, and provides variation in color under illumination, as well as excellent three-dimensional appearance in silhouette; (3) the conventional calcined tile has such a disadvantage that edges thereof are melted by heating to become dull during the unglazed calcination step at 780.degree. C. as well as the glazed calcination step at 1230.degree. C., while the gypsum tile can maintain sharp ridges; (4) it has an excellent acoustic characteristic such as sound insulating and diffusing properties; and that (5) it is non-inflammable. On the other hand, the aforesaid gypsum tile has such disadvantages that (1) the surface hardness thereof is so low that it is easily scratched and contaminated; (2) the impact resistance thereof is so low that sharp edges are easily chipped; (3) it has poor water resistance as well as poor resistance to rainwater and running water; (4) only high-quality .alpha.-type hemihydrate gypsum is primarily used, and such inexpensive by-product gypsum as obtained in the desulfurization of exhaust smoke, in the wet process of phosphoric acid manufacturing, or in the fluorine compound manufacturing is not usable, because discoloration due to impurities causes mottlings; (5) it can not be fixed by use of nails and screws; and that (6) it has low creep resistance.